Molar enthalpies of formation of indium tellurides: In₄Te₃ and InTe

Chemistry Department, Moscow State University,
Moscow B-234, 119899, Russia

(Received 11 March 1995; in final form 23 June 1995)

The standard molar enthalpy of formation of indium monotelluride InTe(cr) was refined and found to be -(71.2 + 0.3) kJ/mol. The standard molar change of energy at the temperature 298.15 K for the reaction: In₄Te₃(cr) + Te(cr) = 4InTe(cr) was determined; the standard molar enthalpy of formation of In₄Te₃(cr) was calculated to be -(235.5 + 1.8) kJ/mol (here and elsewhere in the paper the uncertainties are given for a 95 per cent confidence interval).

1. Introduction

This work is a part of a systematic thermochemical investigation of (indium + tellurium). The molar enthalpy of formation of InTe has been published earlier /1/; here we shall present the results of the determination of the molar enthalpy of formation of tetraindium tritelluride In₄Te₃.

The following phases are believed to exist in {(1-x)In + xTe} over a range of x from 0 to 0.5 /2/: In₂Te, In₄Te₃, In₉Te₇ and InTe. But in accordance with the results of X-ray examination /3/, the crystals, produced by slow cooling of the melts of mixtures {(2/3)In + (1/3)Te} and {(9/16)In + (7/16)Te}, belong to the same substance having the formula In₄Te₃ (the space group is Pnnm, Z = 4, a = 1.5630(3) nm, b = 1.2756(3) nm, and c = 0.4441(2) nm). The analogous structural information has been published for the phase identified in reference 4 as In₂Te, but in reference 5 as In₉Te₇.

In order to eliminate these discrepancies we carried out a special investigation with a view to verifying the reality of existence of the phases: In₂Te, In₄Te₃, and In₉Te₇. The melts {(2/3)In + (1/3)Te} (sample I) and {(4/7)In + (3/7)Te} (sample II) were homogenized by rotation in evacuated quartz ampoules at the temperature T = 950 K, then were quenched at T = 298 K, powdered thoroughly, annealed at T = 673 K during 900 h, and quenched at T = 298 K again. Two phases were detected in the sample I: In₄Te₃ and practically pure In. The sample II consisted of a single phase, identified in the orthorhombic symmetry with the parameters: a = 1.5599(3) nm, b = 1.2725(4) nm, and c = 0.4436(1) nm. These values are in a close agreement with those reported in reference 3. An attempt was made to synthesize the phase In₉Te₇ by annealing a thoroughly homogenized mixture of In₄Te₃ and InTe at T = 673 K during 340 h. X-Ray examination of the annealed product was carried out but no new phase was found. To some extent this fact corroborates the existence in {(1-x)In + xTe} of only two phases over a range of x from 0 to 0.5, namely In₄Te₃ and InTe.

Unlike InTe, In₄Te₃ melts incongruently so that the direct synthesis of this compound in a calorimetric bomb is impossible. This called for an alternative approach to the determination of the enthalpy of formation of In₄Te₃, the reaction:

In₄Te₃(cr) + Te(cr) = 4InTe(cr), (1)

2. Experimental

The specimen of In₄Te₃ was prepared from In and Te, which were semiconductor grade and had nominal mass-fraction purities of 0.99999. The synthesis of In₄Te₃ was performed in accordance with the procedure mentioned above in connection with the preparation of the sample II.

The reaction (1) was carried out in a sealed evacuated quartz ampoule filled with powdered In₄Te₃ and Te in stoichiometric proportions. Use was made of an isoperibol calorimeter with electrical microfurnace in the bomb employed earlier /6/. The experimental details have been given /1/. The only difference is the reduction of the duration of the main period of the calorimetric run to 2 h. This was achieved by the removal of several lids covering shields of the microfurnace /6/ and by filling the bomb with argon to a pressure of 2 MPa. Under such conditions the process of regularization of the heat exchange in the bomb was accelerated, the temperature developed in the microfurnace being sufficient for the melting of the reactant mixture and for the rapid proceeding of reaction (1). The durations of the initial and final periods were 2 h each.

After the end of a calorimetric experiment concerned with reaction (1) the bomb was not disassembled but was used for determination of the energy equivalent E of the calorimetric system. In accordance with a routine procedure we had usually made two determinations of E after each calorimetric experiment to control the completeness of reaction in the bomb, but here there was no need for the second determination because in our experiments with indium monotelluride /1/, the fact was established that the reaction of formation of InTe in the calorimetric bomb proceeds completely (i.e. the homogeneous melt is formed). Good reproducibility of the results of the determinations of E in the experiments with In4Te3 in this work is also evidence for the completeness of the reaction (1) in the bomb (the spread of E_ampoule of ampoules due to the variation of their masses was negligible - not more than 7 J/Ohm).

X-Ray examination of the reaction products was carried out; the lines of InTe only were observed on the X-ray diffraction pattern.

3. Results and discussion

The results of calorimetric experiments on the synthesis of InTe from In₄Te₃ and Te are listed in table I. In this table m₁ and m₂ are the masses of In₄Te₃ and Te respectively, (1 + d) is the ratio of the experimental and stoichiometric amounts of tellurium, q_el is the electric energy supplied to heat the furnace in the bomb, DR is the corrected change of the thermometer resistance (corresponding to the temperature rise) in the main period of the calorimetric experiment, q_r is the energy of the reaction proceeding in the calorimetric bomb, and D_fH_m⁰ is the standard molar enthalpy of formation of In₄Te₃.

It is of particular interest to note that the measured energy of reaction accounts for only a few tenths of a per cent of the total energy introduced into the calorimetric bomb, but the enthalpy of reaction (1) was nevertheless measured with rather good precision.

In this work, use was made of a new copper resistance thermometer, in principle like one described /6/ but more simple in design and reliable in usage. Towards the aim of testing this new equipment we carried out three calorimetric experiments on determination of the enthalpy of formation of InTe. The following values of D_fH_m⁰(InTe,cr) were obtained: (-71.0, -71.5, and -71.8) kJ/mol. These results fall in the same interval of values as those presented in reference 1 so that all the values were treated together. There was obtained finally: D_fH_m⁰(InTe,cr,298.15 K) = -(71.2+ 0.3) kJ/mol. This value is the same as that published in reference 1: {-(71.2 + 0.4) kJ/mol} but the extent of its uncertainty is a little lower.

The enthalpy of formation of InTe_1+d (d is a small excess of Te over the stoichiometric number) was shown in reference 1 to be constant when related to 1*Te, so that the molar enthalpy of formation of In₄Te₃ was calculated in terms of the equation:

D_fH_m⁰(In₄Te₃,cr) = D_rH_m⁰ - (4+d)D_fH_m⁰(InTe,cr),

Just as should be expected the value of (1/3)D_fH_m⁰(In₄Te₃) is higher in magnitude than D_fH_m⁰(InTe) and agrees within the interval of uncertainty with the value -(79.9+ 2.1) kJ/mol which is reported /7/ as the molar enthalpy of formation of In₂Te. This is easy to explain because on the basis of facts presented in the Introduction the authors /7/ really measured the molar enthalpy of formation of {(1/3)In₄Te₃ + (2/3)In}. This is indirect corroboration of the identity of the phase which has been designated earlier as In₂Te to the phase In₄Te₃, otherwise the molar enthalpy of formation of the compound investigated in reference 7 should be higher in magnitude than the molar enthalpy of formation of In₄Te₃ related to 1*Te.

The research described in this publication was made possible in part by Grants No ME9000 and No MT3000 from the International Science Foundation and by Grant No 93-03-5813 from the Russian Foundation of the Fundamental Researches.

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